The invention relates to an ultrasonic sensor for non-invasive detection of gas bubbles in a liquid flowing through a tube, comprising two half-housings capable of being assembled together in which the tube is laid; two ultrasonic transducers facing each other, one acting as transmitter and the other as receiver and one installed in each of the half-housings; and transmission links disposed one in front of each transducer for acoustic coupling to the interposed tube carrying the liquid.
Such ultrasonic sensors are employed wherever a liquid flowing in a pipe or tube must be tested for the presence of gas bubbles, with no intervention in the liquid circuit being allowed during the testing. For example, an extrocorporeal circulation of blood must be continuously monitored with utmost dependability so that no air bubbles, dangerous to the patient, can enter the bloodstream. To detect gas bubbles, the flexible or other tube carrying the liquid is placed between the two ultrasonic transducers. If the tube is completely filled with liquid, the sound waves emanating from the one ultrasonic transducer will be transmitted unimpeded to the opposed transducer acting as receiver. But if gas bubbles traverse the sonic field, the sound waves will be damped thereby, leading to distinct amplitude variations in the received signal.
Ultrasonic sensors operating on this principle are known in a variety of forms. European Published Patent Application No. 53,453, for example, discloses an ultrasonic sensor in which the two mutually opposed ultrasonic transducers are installed in a common one-piece housing. In front of the transducers, elastic transmission links are disposed, between which the tube carrying the liquid can be grasped. There are known devices having two half-housings capable of being applied to the tube from either side, in which case transmission links of elastic material are likewise provided for acoustic coupling (e.g. U.S. Pat. No. 3,663,842). These known ultrasonic sensors have the disadvantage that their use is limited to a single determinate tube diameter, or that their more or less elastic grasping means compress the tube carrying the liquid at the point of measurement. Any local variation in cross section leads immediately to a rise in flow velocity and to a risk of inducing turbulence in the stream of liquid. Consequently, errors of measurement as well as undesired changes in the liquid being monitored can occur. For example, when the liquid is blood there is a danger that blood corpuscles may be destoryed. owing to imperfect contact between the tube and transmission links, furthermore, the acoustic coupling between the ultrasonic transducers and the tube carrying the liquid is unsatisfactory. This problem is only partially remedied by the use of so-called ultrasonic coupling jelly.